Add a custom onclick component to your checkbox:
1. Make the Control ID of the checkbox "yadda" (no dbl quotes)
2. For the checkbox, add a AttributeKeyValuePair custom event to the Custom Events in Functionality / Behavior
Make the event onclick, and add the event handler script as follows:
//BEGIN SCRIPT
var checkbox = document.getElementById('yadda');
var label = document.getElementById('yadda').nextSibling;
while(label.nodeName.toLowerCase()!='label') label=label.nextSibling;
if(checkbox.checked)
label.innerHTML = '*' + label.innerHTML;
else
label.innerHTML = label.innerHTML.substr(1);
//END SCRIPT
Tested in IE8 and it works.